A significant part of the drag of transport aircraft is made up of induced drag. Flying in ground effect close to the ground or water can reduce this drag. Numerous WIGs have been developed and flown.
Several large WIG designs have been proposed but never built. These are summarised in two reports, “Air Cushion Craft Development (First Revision)” (DTNSRDC Report 80/012 (4727 revised) January 1980) by Peter J. Mantle (hereinafter referred to as the Mantle Report), and “Wingship Investigation” (Advanced Research Projects Agency, Sept. 30, 1994) (hereinafter referred to as the ARPA Report).
Because the height of land varies so much it is normal to fly WIGs over water. All existing WIGs fly entirely above the water at the height of the highest wave expected to be encountered plus a margin of safety. This is because of the extremely high wave impact forces that would be incurred at cruise speed. The ARPA Report concluded that designing basic structure and mission loads to tolerate impact with large waves is probably impracticable.
The ARPA Report also concludes that the induced drag increases and the Power Augmented Ram (PAR) lift decreases with the height of the endplates above the water. PAR directs the jet from engines located forward of the wing under the wing to provide added lift at slower speeds. Because of this there is an advantage for WIG endplates to penetrate the waves so that there is no gap at the wave trough between the bottom of the endplate and the water. The existing prior art has not taken advantage of the above as it has been assumed to be impossible to design wave piercing endplates that would (i) have a low enough drag in the water and (ii) be stable at expected angles of yaw at design cruise speed.
As a result, the endplates of existing WIGs usually resemble slender hull shapes similar to high speed racing catamarans, some of which include steps to reduce water friction on take-off. Because these designs are still relatively thick they would incur severe wave impact pressures at cruise speed as well as high drag. Consequently, these endplates are designed to be no lower than the lowest part of the fuselage of the WIG. As a result there is always an air gap greater than the wave height between the wing tip or endplate and the trough of each wave. This restricts their ability to reduce the induced drag. Typical lift/drag ratios of Russian craft are around 18:1 and the ARPA Report study was unable to significantly improve on this figure even for a very large craft of 5,000 tonnes (after making changes required to achieve the longer range set by the study). As these lift/drag ratios are no better than those achieved by aircraft it is understandable why WIGs have never been commercialised.
The WIG configuration that has reached the highest level of technical maturity is the Russian “ekranoplan.” This is described further in the ARPA Report. A typical example of the “ekranoplan” configuration is embodied in the Russian Orlyonok, depicted in FIGS. 1(a) to 1(c). In this prior art WIG, turbofan engines 1 are located on either side of the fuselage 2. These engines 1 are used for underwing blowing PAR to increase the lift of the wing 3 during take-off and landing thereby reducing take-off and landing speeds. The turbo prop engine 4 provides efficient thrust for cruise. The horizontal stabiliser 5 controls the pitching moment. A hydro ski 6 (shown in its lowered position) can be lowered to reduce hull impact pressures on landing. The endplates 7 help contain the pressure under the wing 3 to provide increased PAR lift during take-off and landing. Because the endplates 7 do not extend below the lowest part of the fuselage 2 the effective air gap 8 between the endplates 7 and the water 9 is no less than the gap 10 between the lowest part of the fuselage 2 and the water 9. The ability of the endplates 7 to reduce the induced drag is therefore limited.
FIGS. 2(a) and 2(b) illustrates the side and plan views of the thick prior art endplates 7 of the Orlyonok WIG. On take-off and landing these endplates 7 are designed to plane on the water surface 9 while the fuselage 2 is still supported by the water 9. Steps 12 in the bottom surface of the endplates help this planing action. The sides 13 are contoured to reduce air drag.
The U.S. Navy used thinner endplates with their PAR WIG model experiments disclosed at page 411 of the Mantle Report. These endplates were designed to pierce the waves but were unstable at cruise speed with a moderate angle of yaw. Even if these endplates did not fail, their relatively thick leading edge and forebody would make the drag of these endplates intolerably high when piercing waves at high speed.
In a report entitled “Force and Spray Characteristics of Wing Endplates Penetrating the Water Surface” (General Dynamics/Convair Report GD/C-64-100, April 1964) by W H Barkley (hereinafter referred to as the Barkley Report), four thin endplates with various nose shapes and side configurations are disclosed. Models of these configurations were tested in a towing tank and lift, drag and side forces were measured. When these endplate designs are scaled up to full-scale sizes, the drag forces on the Barkley design are prohibitively high. The raked bottom of three of the Barkley Report designs tested would allow a large air gap and thus cause an increase in induced drag.
FIGS. 3(a), 3(b) and 3(c) provide side elevation, plan and enlarged fragmentary plan views of the thin prior art endplates 14 as disclosed in the Barkley Report and similar to that used on the model in the US Navy experiments referred to in the Mantle Report. As shown in FIG. 3(c) these thin prior art endplates 14 had rounded noses 15 and parallel sides 16. The Mantle report, page 414, concluded that this type of parallel sided, round nosed endplate 14 would fail structurally at cruise speeds.
FIGS. 4(a) to 4(c) depict front, plan and side views of General Dynamics/Convair's test model No. 4 described in the Barkley Report. These endplates 55 have the advantage of a small amount of side force when exposed to moderate amounts of yaw alone (Run No. 5). They do however experience high side forces when certain angles of yaw and roll are combined (Run No. 6). These endplates 55 therefore need to be quite thick to resist the side force resulting in a high drag.
The endplate model No. 4 tested in the Barkley Report had the following dimensions: thickness—1″ (25 mm), working depth—4″ (100 mm), chord length—2′ (610 mm). Scaled up to a depth of 144″ (3.7 m) the dimensions would be: thickness-36″ (914 mm), depth—144″ (3.7 m) and chord length—72′ (22 m). The strength of such an endplate would likely be sufficient but the large thickness would provide excessive drag.
In papers by J. W. Moore, including “Conceptual Design Study of Power Augmented Ram Wing-In-Ground Effect Aircraft” (AIAA Paper 78-1466, Los Angeles, Calif., August, 1978) (herein after referred to as the Moore Report), endplates as depicted in FIGS. 5(a) to 5(c) and based on model No. 4 of the Barkley Report were proposed. Because of their high drag, these endplates 57 were designed to operate above the water most of the time except for impact with every 1/1000 wave crest to a depth of 0.63′ (192 mm) and 1.4′ (427 mm) for sea states 3 and 4 respectively. The drag force of each endplate 57 was calculated as 687,000 lbs (3.06 MN) and 1,148,000 lbs (5.11 MN) at an immersion depth of 1.4′ (427 mm) and yaw angles of 0 degrees and 10 degrees respectively. For the two endplates 57 the total drag would be 1,374,000 lbs (6.112 MN) at 0 yaw and 2,296,000-lbs (10.21 MN) at 10 degrees yaw, equivalent to 88% and 147% of the gross weight of the entire WIG (for an immersion depth of only 1.4′ (427 mm)). This extremely high drag is caused by the thick wedge nose 58 chosen “to assure non-attached flow along the endplate length” when impacting every 1/1000 wave crest. The endplates 57 are not designed for and would be completely impracticable for continuous immersion to the depth of the wave trough, as the drag would be sufficient to down the WIG. In addition Moore concluded that the high side force in yaw would create structural failure of the endplate 57 at an immersion depth of 4.3′ (1300 mm) and a speed of 265 knots (136 m/s) in a sea state 4.
The prior art endplates discussed above generally have very high drag characteristics and lack of stability if immersed in water.
In two further reports, “On the Minimum Induced Drag of Ground Effect Wings,” (The Aeronautical Quarterly, Royal Aeronautical Society, London, UK, August 1970) by P. R. Ashill (hereinafter referred to as the Ashill Report) and “Wind-Tunnel Investigation of Single and Tandem Low-Aspect-Ratio Wings In Ground Effect” (Lockheed Calif., March 1964) (hereinafter referred to as the Lockheed Report), it was shown that the addition of vertical plates at each end of the wing can be used to reduce or eliminate the induced drag.
Ashill concludes that the induced drag→0 as l/b→h/b (where l=distance from the bottom edge of the wing at the ¼ chord point to the bottom of the endplate, h=distance between from the bottom edge of the wing at the ¼ chord point to the ground and b=span of wing). This is confirmed by the following extrapolation of the results found in FIGS. 17 and 18 of the Lockheed Report, reproduced here as Tables 1 and 2 wherein:
Cl=lift coefficient, L/D=lift/drag ratio, AR=aspect ratio of the wing, h=distance between bottom of the endplate and the ground; S=area of the wing and O.G.E.=2-dimensional test Out Of Ground Effect.
TABLE 1Flat Endplates, AR = 4, Endplate depth = 0.15 chordInduced Drag/Cl/(L/D)h/√{square root over (S)}Total DragInduced Drag√{square root over (()}h/√{square root over (S)})0.5/50O.G.E.0.010.0—0.5/380.010.013160.003160.03160.5/350.020.014290.004290.03030.5/300.040.016670.006670.0333
TABLE 2Contoured Endplates, AR = 4, Endplate depth = 0.015 chordInduced Drag/C1/(L/D)h/√{square root over (S)}Total DragInduced Drag√{square root over (()}h/√{square root over (S)})0.5/56O.G.E.0.008930.0—0.5/400.010.01250.003570.03570.5/360.020.01390.004970.03550.5/310.040.0160.0070.035 
These figures show that, for small values of h/√{square root over (S)}, the induced drag/(h/√{square root over (S))}≈constant. Thus the Induced Drag approaches zero as h approaches zero.
The application of this concept can effectively raise the elevation of the basic structure so as to avoid its impact with waves. Thus if endplates could be designed with adequate structural strength and low enough drag to operate immersed in the water, a WIG with attractive performance could be achieved.